Well, people back then didn't know what would work and what wouldn't, and they didn't really consider the possibility of advancements in the field of tank design in the enemy army either, so why not a flying tank? When it was concieved, there were no knowledge of the problems it would face a few years later if it had been used.
As for the concept as a whole however, flying tanks aren't that good of an idea since they can't carry the armor needed to actually work as tanks, 15mm armor is about the same ammount that you'd expect from an armoured car which can also be made lighter due to the lack of tracks and heavy tank gearbox/suspension, on top of that, a 75mm gun can easily be fitted on one of those.
Long story made short, the tank would have had too weak armour to withstand heavy machinegun fire, too weak weapons and the ability to go at 100mph doesn't make you a hard target to hit when you have to stop and aim in order to hit anything (In real life, tanks usually don't ram each other).